The present invention relates to a process for producing a solid-state radiation detector by assembling unitary elementary slabs in order to obtain a composite slab of large size, and it more particularly relates to a process for adhesively bonding these unitary elementary slabs.
The invention also relates to a detector thus produced.
The principal application of such detectors is X-ray radiology. To give a concrete idea, in what follows, without limiting its scope in any way by this, the context of the preferred application of the invention will be assumed, unless otherwise indicated.
A scintillator according to the prior art is described by way of nonlimiting example, in French patent application FR-A-2 636 800 (THOMSON-CSF).
The mode of operation and the general structure of a solid-state X-radiation detector will now be sumarized with a reference to the description of FIGS. 1a to 3 appended [lacuna] the present description.
According to the state of the art, radiation detectors are produced on the basis of one or more matrices of solid-state photosensitive elements. Known solid-state photosensitive elements are not directly sensitive to rays with very short wavelengths, as X-rays are. It is necessary to combine them with a scintillator component. The latter is made of a substrate which has the property, when it is excited by X-rays, of emitting light in a range of longer wavelengths: in the visible spectrum (or the near visible spectrum). The particular wavelength depends on the substance used. The scintillator therefore acts as a wavelength converter. The visible light thus generated is transmitted to the photosensitive elements which carries out photoelectric conversion of the received light energy into electric signals which can be processed by suitable electronic circuits.
FIGS. 1a and 1b represent two mutually orthogonal lateral sections of a matrix of photosensitive elements which is conventionally combined with a sheet of scintillating substance.
Each photosensitive element has a photodiode or a phototransistor which is sensitive to photons in the visible spectrum or the near visible spectrum. By way of example, as illustrated in FIGS. 1a to 1d, each photosensitive element consists, for example, of two diodes Dmn1 and Dmn2 which are arranged head-to-tail, and the matrix array RM has column conductors, Cc1, to Ccx, and row conductors, Cl1 to Cly. Each of the diodes Dmn1 and Dmn2 constitutes, in a known way, a capacitor when it is reverse-biased. The first diode Dmn1 has a capacitance typically ten times smaller than the capacitance of the second diode Dmn2. It principally fulfills the function of a switch, whereas the second diode is preferably photodetecting.
At each intersection of a row and a column, for example of row Cln and column Ccm (see FIG. 1d), such a set of two diodes head-to-tail, Dmn1 and Dmn2, is arranged. The diodes can be replaced by transistors produced in xe2x80x9cTFTxe2x80x9d technology, TFT standing for thin film transistor.
The conductors 12 (FIGS. 1a and 1b) consist of a metal deposit on an insulating substrate 10, preferably glass. The deposition is followed by a photoetching operation in order to obtain parallel conductive tracks of suitable width. The diodes (for example Dmn1 and Dmn2) are formed by deposition, on the column conductive tracks 12, then etching, of the layers of amorphous silicon (ASi) which is intrinsic or doped using P or N type semiconductor material. A very thin layer of conducting, preferably transparent material is deposited on the insulating layer 20 in order to form, after etching, the row conductive tracks 22 of the matrix array RM.
The assembly described above forms what is generally referred to as the unitary elementary xe2x80x9camorphous silicon slabsxe2x80x9d.
The row conductors Cl1-Clx and the column conductors Cc1-Ccy constitute the electrodes for biasing the diode capacitors. The latter store electric charges when they are subjected to light radiation and deliver an electric signal, proportional to the stored charge, when they are electrically biased. The row conductors Cc1-Ccx and the column conductors Cc1-Ccy are addressed in a suitable chronological sequence, so that all the pixels pmn are biased sequentially in a predetermined order. The signal delivered by each pixel pmn is thus recovered and processed by electronic circuits (not shown) so as to reconstruct (point by point) the image stored in the form of electric charges.
The signals are recovered in respective connection zones 3 and 4, for the rows Cl1-Clx and the columns Cc1-Ccy. The connections with the external electronic circuits may be made using flexible multiconductor cables, 30 and 40, respectively.
It is generally necessary to provide sequences referred to as xe2x80x9coptical relevellingxe2x80x9d of the pixels pmn, once the signals have been delivered by them. The chronological sequence of the addressing signals is adapted accordingly. Optical relevelling sequences are inserted between the reading signals. They consist in performing generalized illumination of the pixels pmn, after reading. The purpose of these sequences is to re-establish an electrical reference state on the pixels pmn which have been perturbed during the phases of storing and reading the charges.
This generalized illumination is performed via the rear face of the glass slab 10, which must be sufficiently transparent at the wavelengths of the light which is used.
As indicated above, the photosensitive elements need to be illuminated with visible light (or in a range close to visible light). It is necessary to provide a scintillator which converts the X-rays into light energy, in the visible-wavelengths spectrum. To that end, it is sufficient to cover the amorphous silicon slab described above with a layer of scintillating substance 24. By way of example, for a detector sensitive to X-rays of the order of 60 KeV [sic], the scintillating substance used is cesium iodide (CsI) doped with sodium iodide (NaI) or thallium iodide (TiI), depending on whether the intention is to obtain a light signal of wavelength 390 nm or 550 nm, respectively.
The amorphous silicon slab which has just been described is produced by vacuum evaporation of thin films of material on the glass slab. The dimensions of the glass slab must be compatible with the current dimensional capacities of machines for carrying out the deposition.
However, the need has been felt to provide slabs with large sizes, these sizes being incompatible with the aforementioned deposition machines. It is thus necessary to resort to unitary elementary slabs, of smaller sizes, which are assembled by juxtaposition with one another. By way of nonlimiting example, a chequer-board of four elementary unitary slabs is assembled in order to form a composite slab of large size. Such an assembly process is described, for example, in French patent application FR-A-2 687 494 (THOMSON TUBES ELECTRONIQUES). The unitary slabs preserve their autonomy as regards the addressing of their own pixels pmn .
FIGS. 2a and 2b appended to the present description illustrate such an assembly, respectively in side and plan view. In the example described, the composite slab of large size comprises four unitary elementary slabs 10a to 10d.
The unitary elementary slabs 10a to 10d are cut precisely on two sides of their periphery which are free of the connection zone (not shown) so that the active zones of pixels, RMa to RMd, are flush with the edge of the cut slab. The cut slabs 10a to 10d are then positioned relative to one another in order to preserve continuity of the active zone of the pixels and their pitch, from one slab to the next.
The assembly is carried out by adhesive bonding (film 6) of a common support 7 on the cut and positioned slabs 10a to 10d. This support 7 must also be transparent enough to visible light in order to allow optical relevelling of pixels of all the unitary slabs thus assembled.
The simplest method for producing the scintillator is to deposit a layer of doped caesium iodide (CsI) on any desired substrate, to anneal this substrate in order to obtain the desired luminescence properties and to attach this scintillator-substrate assembly against the adhesively bonded assembly. The scintillator may be attached or coupled optically to the slab by adhesive bonding.
The performance obtained with a scintillator produced in this way is, however, mediocre especially in terms of isolation. This is because refraction is observed of the visible light output by the scintillator, either in the thickness of the adhesive, in the case of coupling to the slab, or in the thickness of the air layer which is difficult to control in the case of pressing onto the slab.
Another method for producing the scintillator consists in producing the scintillator by direct evaporation of a scintillating substance onto the composite slab obtained by assembling and adhesively bonding the elementary slabs 10a to 10d. This solution has the advantage of a scintillator in intimate contact with this composite slab. The scattering of light at the scintillator/slab interface and the loss of resolution which results from this are minimized.
The adhesively bonded assembly thus obtained will have to experience environmental, thermal and mechanical stresses. It must withstand impact vibrations and jolts. The film of adhesive 6 must be flexible enough to withstand these mechanical stresses. In order to allow the pixels pmn to be optically relevelled via the rear face of the slabs 10a to 10d, the film of adhesive 6 must furthermore be transparent enough to the visible light used for this optical relevelling.
Lastly, when the scintillator is produced by direct evaporation, the adhesively bonded assembly undergoes luminescence annealing of the caesium iodide layer (CsI). The film of adhesive must therefore withstand the annealing temperature. It must be also be flexible enough to withstand the difference in coefficients of expansion of the materials to be adhesively bonded (unitary slabs 10a to 10d and common support 7) which are a priori different.
In order to satisfy these requirements, use may be made of a two-component silicon resin which polymerizes by polyaddition, in order to carry out the adhesive bonding.
The multislab detectors of large size which are produced according to the process which has just been summarized, therefore comprise the following steps, illustrated schematically with reference to FIG. 3 appended to the present description:
a/ unitary elementary slabs 10a to 10d (only two of which, 10a and 10b, can be seen in FIG. 3) are positioned relative to one another on a substrate 8 (referred to as a positioning substrate): the pitch of the pixels pmn must be respected, the inter-slab spaces e typically lying in the 10 to 100 xcexcm range;
b/ a film of adhesive 6 is spread over the assembly thus formed;
c/ the common support 7 is then fitted on the assembly.
This process does, however, have drawbacks. Specifically, because of the fluidity of the adhesive, it most often penetrates between the unitary elementary slabs (in the spaces e) and spreads over their faces which are covered with the semiconductor deposits (for example the matrix arrays RMa and RMb which can be seen in FIG. 3).
This problem must be avoided. The degradation of the clean condition and the topography which result therefrom are incompatible with the deposition of the scintillating substance over all the unitary elementary slabs, either in the case of an attached scintillator or in the case of a scintillator produced by direct evaporation.
The subject of the invention is to overcome the drawbacks which result from the processes for producing detectors according to the prior art.
To that end, the adhesive bonding according to the process of the invention is carried out in two stages. In a first stage, the inter-slab spaces are sealed by a first type of adhesive, without filling them entirely. In a second stage, the adhesive bonding proper is carried out by using a second type of adhesive.
The invention therefore relates to a process for producing a radiation detector consisting of the combination of a photosensitive component and a scintillator, the said photosensitive component consisting of at least two unitary elementary slabs, each unitary elementary slab containing a plurality of active elements or pixels and the edges of two adjacent slabs being separated by a determined inter-slab space, the process comprising at least the following phases:
a/ juxtaposition of the said unitary elementary slabs on a positioning substrate in order to form a composite slab of larger size whose unitary elementary slabs are separated from one another by the said determined inter-slab space, the said plurality of active elements or pixels being placed in contact with the positioning substrate;
b/ adhesively bonding the said composite slab onto a common support;
characterized in that the said adhesive-bonding phase comprises, in sequence:
a first adhesive-bonding step consisting in sealing the said determined inter-slab spaces by forming a bead of adhesive, leaving a free zone between its lower end and the said positioning substrate;
and a second adhesive-bonding step consisting in spreading a film of adhesive covering the said unitary elementary slabs and the said bead of sealing adhesive.
Advantageously, the adhesive used in the first step is a high-viscosity adhesive (under ambient temperature and pressure conditions) and the adhesive used in the second step is an adhesive with much lower viscosity (an adhesive which is fluid under the deposition conditions). The bead of adhesive deposited in the first step can thus fulfil the function of a plug filling the upper part of the gaps between slabs, and the second layer of adhesive does not penetrate into these gaps.
The invention also relates to a radiography detector produced according to this process.